Electrical apparatus



NOV. 26, 1940 DEVQL AL 2,222,796

ELECTRICAL APPARATUS Filed Dec. 2, 1938 Z;v I lI Load A -n=-c1 we? INVENTORS Lee Decal and Bicha d A. Ham e29.

ZHEIQ ATTORNEY Patented Nov. 26, 1940 PATENT OFFlfiE ELECTRICAL APPARATUS Lee Devol and Richard A. Hartley, PennTownship, Allegheny County, Pa., assignors to The Union .Switch & Signal Company, Swissvale, Ta, a corporation of Pennsylvania.

Application December 2, 1938, Serial No. 243;620

2 Claims.

Gur invention relates to electrical apparatus, and particularly to filters-of the mechanically tuned type.

It is frequently desirable in electrical appa- :r-atus .to :cause an electroresponsive device to :respond to electrical energy ofone frequencyonly. The most commonly .usedmeans forachieving this result is a filterof the electromagnetic type.

such a filter isused for verylow frequencies, however, it becomes cumbersome and has a tendency to lose its sharpness oftuning, that islto say, its ability to distinguish between frequencies differing from each other by only a small percentage.

It is generally known .that a mechanically tuned vibrator is more sharply resonant nthanlone stage of an electrical filter at the same frequency. 1011! present invention takes advantage of this fact to provide a filter which will be more highlyselective than the usual electromagnetic filter with:the.same..number of stages, or a filterthat will produce equally good resuitswith a simpler, less cumbersome and less expensive construction.

We'sh-all describe two .forms of electrical-apparatus embodying our invention, and shall then point out .the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a diagrammatic view showing one form of apparatus including a mechanically tuned filter embodying our invention. Fig. 2 is a view .showing another form of apparatus including another form of mechanically tuned filter embodying our invention.

Similar reference characters refer to similar parts in both views.

Referring first to Fig. 1, the filter here illustrated, which filter is designated as a whole by the reference character F, is adapted to pass a single frequency only, and to operate over a wide range of impedances. The filter comprises a suitable source of magnetomotive force, here shown as a permanent magnet 3, the ends of which terminate in pole pieces 4 and 5 of high permeability ferromagnetic material. The pole pieces 4 and 5 are substantially C-shaped, and are arranged to provide two parallel air gaps 6 and 1 across which the flux from the permanent magnet passes in substantially equal amounts and in the same direction. Cooperating with the air gaps 6 and 1, respectively, are armatures 8 and 9 secured to one end of a spring reed 10, the other end of which is fastened to a fixed support II. The filter also comprises a winding in the form of two coils l2 and 13 "disposed 'onopposlte legs of the pole piece 4 and connected in series in such manner that currents flowing therein will setup additive fluxes in the pole pieces. The parts are so arranged and so proportioned that the reed f0 and associated armatures are mechanically tun-ed to vibrate at the single frequency which it is desired to have the filter pass, and that when the reed is vibrating, the vibration will not materially change the clearances between the armatures and the pole pieces.

With the filter constructed in this manner, 10

whenever an alternating voltage is impressed across the coils l2 and IS, the resultant flux which is induced in the pole piece 4 will cause the flux density in the air gap '6 to increase and 1-5 that in the air gap 1 to decrease during onehalfof each cycle, whereas during the next half of each cycle the flux in the air gap 6 will decrease and that in the air gap 1 will increase.

air gap 1 will exceed that on armature 8 tending to pull it into the field in the air gap 6, with the result that the reed I 0 will tend to vibrate. If the applied electromotive force is of the same frequency as that to which the reed and the associated 'armatures are tuned mechanically, the reed ID will vibrate through a considerable amplitude, and will cause the permanent magnet flux threading coils l2 and I3 to vary in magnitude at the same frequency as the reed is vvibrating, thereby generating a counter-electromotive force in these coils, and hence causing the coils to offer a relatively high impedance due to the motion of the reed. If,'however, the frequency of the applied electromotive force is different from thatto which the reed is tuned, there will be very little motion of the from that for which the filter is tuned, the low impedance offered by the filter will cause a relatively large voltage drop across the impedance I, and under these conditions the voltage impressed across the load will be insufi'icient to efiectively operate it. When, however, the frequency of the current supp-lied by the alternator A is the same as that for which the filter is tuned, then the high impedance ofiered by the filter will cause less voltage drop across the impedance I, with the result that the voltage impressed across the load will rise to a value which is sufiicient to effectively operate it.

One advantage of a filter constructed as shown in Fig. 1 is that it possesses an extremely sharp resonance, and is less cumbersome at low frequencies than the ordinary reactor-condenser combination for effecting the same result.

It should be pointed out that if the source A is of the*type which has a limited output, by properly proportioning the parts the impedance I may be dispensed with.

Referring now to Fig. 2, the filter Fl here shown comprises two vibrating reeds l4 and I5 secured at one end to the opposite ends of the core l6 of an electromagnet E, the winding I! of which is constantly supplied with direct current from a suitable source here shown as a battery B. The reeds M. and [5 are mechanically tuned to vibrate at the resonant frequency at which it is desired to have the filter operate, and cooperate intermediate their ends with a core 18 of high permeability magnetizable material, and at their free ends with a core l9, also of high permeability magnetizable material, sufiicient air gaps being provided between the cores l8 and 29 and the reeds to prevent the reeds from striking the cores even when vibrating at their maximum amplitude. The core I8 is surrounded by an input winding 20 which is connected with the alternator A, and the core I9 is surrounded by an output winding 2| which is connected with the load L.

With the filter constructed in the manner shown in Fig. 2, the flux due to the electromagnet follows the path indicated by the long arrows, while the greater portion of the flux due to the alternating current supplied to the winding l8 follows the path indicated by the short arrows, it being obvious, of course, that this latter flux will tend to traverse this path in opposite directions during alternate half cycles. When the alternating fiux is in the direction in which the short arrows are pointing, the alternating and permanent magnet fluxes will tend to add in the air gaps 22 and 23 between the core l8 and the reeds l4 and I5, and, under these conditions, the reeds will be drawn toward each other more strongly than would be the case if the alternating flux were absent. When, however, the alternating flux is in the opposite direction to that in which the short arrows are pointing, the two fluxes will then tend to buck each other in the air gaps 22 and 23, and, under these latter conditions, the reeds are attracted less strongly than would be the case if the alternating flux were absent. It will be seen, therefore, that when alternating current is being supplied to the winding 2|], the reeds l4 and I5 will tend to vibrate at the frequency of the alternating current, but that this vibration will be extremely small except when the frequency of the alternating current is equal to that for which the reeds are tuned. It will also be seen that when the reeds are vibrating, a cyclic variation will take place in the flux from the electromagnet which traverses the core 19, so that an alternating voltage will be induced in the winding 2| and will be supplied to the load L. In this connection it should be noted that since the electromagnet E has a high permeance and the magnetic circuit through the core l9 has a low permeance, when the reeds are not vibrating, substantially none of the alternating flux will thread core l9, and hence no appreciable electromotive force will be generated in the winding 2| under these conditions.

Obviously a permanent magnet can be substituted for the electromagnet E in the filter Fl if desired, although this will result in lower values of operating flux and smaller amplitudes because of the lower permeance of permanent magnet materials.

Furthermore, since the windings 20 and 2| with the accompanying magnetic circuits are inductive, the advantages of electrical tuning in addition to mechanical tuning may be obtained in both the input and output circuits by connecting suitable condensers Cl and C2 in these circuits. As here shown, the condensers CI and C2 are connected in parallel with their respective windings, but in some cases better results can be obtained with series tuning.

It will be obvious, of course, that the output 0 the filter Fl can be used to actuate a second similar filter etc. to provide as many stages as may be desired.

One advantage of a filter embodying our invention over the usual type of vibrating device is that there are no contacts which can become worn out and cause failure of the apparatus.

Although we have herein showm and described only two forms of electrical filters embodying our invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. An electrical filter comprising means for creating a unidirectional magnetic field which normally divides between two parallel paths each including an air gap, two magnetically independent armatures secured to a cantilever reed and cooperating with said two air gaps respectively, said reed and said armatures being tuned mechanically to vibrate at a particular frequency, and a winding associated with said two paths and effective when supplied with alternating current to cause the flux to alternately vary in said two air gaps, whereby said winding will offer a relatively high impedance to alternating current having a frequency corresponding to that to which said reed is tuned and a relatively low impedance to alternating current of any other frequency.

2. An electrical filter comprising a permanent magnet, two pole pieces secured to the ends of said permanent magnet and forming two parallel magnetic paths each including an air gap, a pair of magnetically'independent armatures secured to a vibratory cantilever reed and cooperating with said two air gaps, said reed and said armatures being tuned mechanically to vibrate at a particular frequency, and a winding disposed on one of said pole pieces and effective when supplied with alternating current to alternately vary the flux traversing said air gaps.

LEE DEVOL. RICHARD A. HARTLEY. 

